Free space focusing of terahertz light is normally achieved through the use of bulky parabolic mirrors. Alternatively, for focusing onto a substrate or sample, polished high resistivity silicon lenses are commonly used. This paper presents the design, fabrication and testing of an alternative approach, based on Fresnel microlenses which have been optimised for use in the terahertz region. The microlenses are fabricated using layers of SU-8 photoresist and conventional UV photolithography. The lens design approach presented here provides a low cost, mass production ready alternative to silicon lenses. Fresnel lenses can have a large numerical aperture and a short focal length and are well suited for use in terahertz imaging systems. The focal point of the demonstrated Fresnel microlens has been calculated to be approximately 5 mm at 1 THz using a commercial FDTD solver, Lumerical. Characterization of the microlenses by VNA (Vector Network Analyzer) operating in the frequency range of 750 GHz to 1.1 THz is presented and discussed. The measured focal length using the VNA approach corresponds well to the values calculated using the FDTD solver and demonstrates effective focusing from highly compact lenses.
Periodic nanostructure arrays have been ubiquitously exploited lately due to their properties and prospective applications in production of templates for self-induced and gold (Au)-catalysed nanowires (NWs), because this approach is relatively cheap, time-efficient and do not require electron beam lithography. The technique consists creating nanoholes in SiO<sub>2</sub> to expose the silicon Si (111) beneath where self-induced NWs can nucleate, while nanodots deposited onto the Si (111) surface serve as catalyst seeds. For Au-catalysed NWs, a monolayer of self-assembled polystyrene nanospheres (PNS 300nm) was created on a 2 inch Si wafer by spin coating and later etched for a short time before a very thin Au-catalyst layer was deposited. In turn, for self-induced, PNS monolayer was created onto a SiO<sub>2</sub>-Si substrate. A longer etch was required to reduce PNS diameter significantly to leave relatively larger spacing where chromium is blanket deposited. PNS were lifted off by sonicating the samples in toluene produce the periodic arrays of nanodots and nanoholes, respectively. The underlying SiO<sub>2</sub> was etched further through the nanoholes to uncover the Si below. 200 nm holes and 30-70 nm dots were demonstrated through the bespoke methods. The patterned substrates served as master templates, subsequently copied using polydimethylsiloxane (PDMS) to produce a flexible stamp for nanoimprint lithography. A bilayer resist lift off process was developed to print the replicated nanodots or nanoholes on large-area substrates onto which III-V NWs can be grown.
A structurally consistent, high aspect ratio nanopores template, featuring 110 nm diameter, 110 nm deep pores have been
fabricated and used as a shadow mask to evaporate multilayer nano OLED cylinders based on Iridium organic-metal
complex emitters on ITO/glass. We have characterised the nanostructures using atomic force microscope (AFM) and
scanning electron microscope (SEM) images. The emissive properties and electrical characteristics of the nano-device
are presented. Luminescence efficiency and power efficiency have been studied and compared with conventional large
This paper reports on the development of micromachined pillar arrays for the filtering of terahertz radiation. These pillar
arrays are fabricated using ultraviolet based processing of thick SU8. This micromachining technique enables the array
patterns, dimensions, and consequently the filter characteristics, to be readily defined. In particular, we demonstrate that
by combining individual filter arrays with either different periods or pillar diameters we can isolate individual pass bands
in the 1 to 2 THz region.
Terahertz (THz) spectroscopy of a biomolecule with spatial resolution below the diffraction limit of the radiation has
been achieved by use of an all-optical, contactless transient mirror technique. A resolution of around 50 &mgr;m is
determined by the use of a test sample of gold strip lines deposited on GaAs, and the differential THz time-domain
spectroscopy (THz-TDS) response of biotin has been measured in both the presence and absence of the transient mirror
at room temperature. These preliminary results demonstrate the potential for use of the technique for the chemical
identification and characterisation of biomolecules in small volumes with the ultimate goal being microscopic imaging of
live cells. The technique may find applications in quality control for semiconductor processing, and in identifying
material imperfections, i.e. small cracks in non-destructive testing. We discuss the limitations of the transient mirror
technique along with several advantages over other related techniques.
We report on the development of a surface micromachined process for the fabrication of coaxial apertures surrounded by periodic grooves. The process uses a combination of copper electroforming and the negative epoxy based resist, SU8, as a thin flexible substrate. The device dimensions are suitable for the implementation of filters at THz frequencies, and measurements show a pass band centred around 1.5 THz. These devices could form the basis of the next generation of THz biosensors.
At the present time the interaction of Terahertz (THz) radiation with random structures is not well understood. Scattering effects are particularly relevant in this spectral regime, where the wavelength, and the size and separation of scattering centres are often commensurable. This phenomenon can both be used to advantage in imaging and sensing, but conversely can have adverse effects on the interpretation of a "fingerprint" spectrum. A new mathematical method, the <i>Phase Distribution Model</i>, is reported here for the calculation of attenuation and scattering of THz radiation in random materials. This uses a Phase Distribution Function to describe the effect of the non-absorbing scatterers within the media. Experimental measurements undertaken using previously published results, data obtained from specially constructed phantoms and from everyday textiles have been compared with the theory. These experimental results encompass both cylindrical and spherical scattering situations. The model has also been compared with exact calculations using the <i>Pendry code</i>.
A new mathematical method, the <i>Phase Distribution Model</i>, is devised for the calculation of attenuation and scattering of THz radiation in random materials. The accuracy of the approximation is tested by comparison with exact calculations and with experimental measurements on textiles and specially constructed phantoms.